There was a time — 1972, to be precise — when a single chess match held the world spellbound: The seven-game championship between Boris Spassky, representing the Soviets’ decades-long stronghold on chess mastery, and the neurotic American prodigy Bobby Fischer. This Cold War in miniature made Fischer a national hero and chess a new national pastime; in her highly acclaimed documentary, Liz Garbus details the wind-up to the match, its nail-biting twists and turns, and the long, sad, paranoid decline that followed all the way to Fischer’s death in 2008. It screens two nights only at The Belcourt this week as the theater kicks off its “Doctober” documentary festival.